U.S. patent application number 09/774238 was filed with the patent office on 2002-02-21 for laser with versatile output energy.
Invention is credited to Gehrke, Michael, Schroeder, Thomas.
Application Number | 20020021730 09/774238 |
Document ID | / |
Family ID | 26874489 |
Filed Date | 2002-02-21 |
United States Patent
Application |
20020021730 |
Kind Code |
A1 |
Schroeder, Thomas ; et
al. |
February 21, 2002 |
Laser with versatile output energy
Abstract
A method of operating a laser system for increasing the
lifetimes of optical components of the resonator includes the steps
of configuring the laser system to initially output laser pulses at
an energy in a range above a predetermined energy for industrial
lithographic processing, and attenuating the energy of the output
pulses to the predetermined energy. A further step includes
reducing an amount of attenuation as optics of the laser resonator
age to maintain the laser pulses at the predetermined energy, or
reducing the amount of attenuation to produce pulses having a
higher energy than the predetermined energy.
Inventors: |
Schroeder, Thomas;
(Goettingen, DE) ; Gehrke, Michael; (Kalefeld,
DE) |
Correspondence
Address: |
Andrew V. Smith
Sierra Patent Group, Ltd.
P.O. Box 6149
Stateline
NV
89449
US
|
Family ID: |
26874489 |
Appl. No.: |
09/774238 |
Filed: |
January 29, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60178620 |
Jan 27, 2000 |
|
|
|
Current U.S.
Class: |
372/57 |
Current CPC
Class: |
G03F 7/70041 20130101;
H01S 3/08009 20130101; H01S 3/005 20130101; G03F 7/70025 20130101;
H01S 3/1061 20130101; H01S 3/104 20130101; H01S 3/2251 20130101;
H01S 3/225 20130101 |
Class at
Publication: |
372/57 |
International
Class: |
H01S 003/22 |
Claims
What is claimed is:
1. An excimer or molecular fluorine laser system, comprising: a
discharge chamber filled with a gas mixture including molecular
fluorine and a buffer gas; a plurality of electrodes within the
discharge chamber and defining a discharge region therebetween
connected to a pulsed discharge circuit for applying discharge
pulses to the electrodes for energizing the gas mixture; a
resonator including the discharge chamber and a line-narrowing
module for generating a line-narrowed laser beam; and an attenuator
for reducing an energy of the laser beam to a predetermined energy
for lithographic processing.
2. The laser system of claim 1, wherein said attenuator is
configured to reflect a first portion of the beam to a diagnostic
module for monitoring one or more parameters of the beam, and
wherein a second portion of said beam transmits said attenuator as
an output beam of said laser system.
3. The laser system of claim 2, wherein said diagnostic module
includes an energy detector for detecting an energy of the
reflected beam.
4. The laser system of claim 2, wherein said diagnostic module
includes a spectrometer for determining a spectral parameter of the
reflected beam.
5. The laser system of claim 2, wherein said diagnostic module
includes a detector for measuring a parameter of the reflected beam
selected from the group of parameters consisting of temporal pulse
shape, spatial profile, beam alignment, beam divergence and
spectral distribution
6. The laser system of claim 2, wherein said diagnostic module
includes an inspection window.
7. The laser system of claim 1, wherein said attenuator includes an
attenuator plate for reflecting away a first portion of the beam
and for transmitting a second portion of said beam as an output
beam of said laser system, and wherein said laser system further
comprises a compensator plate for compensating a shift of the beam
produced by refraction through said attenuator plate.
8. The laser system of claim 1, wherein said attenuator is disposed
outside of said resonator of said laser system and said attenuator
reduces the energy of an outcoupled laser beam from said
resonator.
9. The laser system of claim 8, said attenuator for reflecting a
first portion of the beam to a diagnostic module for monitoring one
or more parameters of said beam selected from the group of beam
parameters consisting of energy, bandwidth, wavelength, temporal
pulse shape, spatial profile, beam alignment, beam divergence and
spectral distribution
10. The laser system of claim 8, wherein the discharge circuit is
configured to operate in a range between a minimum voltage and a
maximum voltage, and wherein when the minimum voltage is applied
across the electrodes, the energy of the outcoupled laser beam
before impinging upon the attenuator is greater than the
predetermined energy.
11. The laser system of claim 8, wherein the attenuator is disposed
in a housing flushed with an inert gas.
12. The laser system of claim 11, wherein the inert gas is selected
from the group of inert gases consisting of nitrogen and argon.
13. The laser system of claim 8, wherein the attenuator is driven
by a motor for adjusting the attenuation.
14. The laser system of claim 1, wherein said attenuator includes
an attenuator plate for reflecting away a first portion of the beam
and for transmitting a second portion of said beam as an output
beam of said laser system.
15. The laser system of claim 1, wherein the attenuator has
variable attenuation for reducing several output pulse energies of
the outcoupled laser beam each to said predetermined pulse
energy.
16. The laser system of claim 15, wherein said several output pulse
energies are energies of output pulses produced by application of a
same input voltage from said discharge circuit to said electrodes
at different stages of aging of optics of said line-narrowing
module.
17. The laser system of claim 15, wherein the attenuator includes a
plurality of attenuator plates for reflecting a portion of the
output energy of pulses of the outcoupled laser beam, each plate
having a different reflectivity.
18. The laser system of claim 15, wherein said attenuator includes
a slot for insertion of any of a plurality of attenuator plates for
adjusting the variable attenuation of the attenuator.
19. The laser system of claim 15, wherein said attenuator includes
a rotatable holder for holding a plurality of attenuator plates for
reflecting a portion of the energy of pulses of the outcoupled
laser beam, each plate being rotatable into the path of the laser
beam for selecting an amount of attenuation depending on the pulse
energy of pulses of the outcoupled laser beam and a value of the
predetermined energy.
20. The laser system of claim 1, wherein the attenuator includes
one or more attenuator plates arranged at Brewster's angle.
21. The laser system of claim 1, wherein the attenuator includes
one or more attenuator plates each including a reflective
coating.
22. The laser system of claim 1, wherein the attenuator includes
one or more attenuator plates, and wherein a reflectivity of the
one or more plates is polarization coupled.
23. The laser system of claim 1, wherein the discharge circuit is
configured to operate in a range between a minimum voltage and a
maximum voltage, and wherein when the minimum voltage is applied
across the electrodes, the energy of laser pulses before impinging
upon the attenuator is greater than the predetermined energy.
24. A method of operating an excimer or molecular fluorine laser
system including a discharge chamber filled with a gas mixture and
having a plurality of electrodes therein connected to a pulsed
discharge circuit, and including a laser resonator for generating
pulsed output beam, said method for increasing lifetimes of optical
components of the resonator, and comprising the steps of:
configuring the pulsed discharge circuit to apply electrical pulses
to the electrodes of the laser system to output laser pulses
initially having an energy in a range above a predetermined energy
for industrial lithographic processing; and attenuating said energy
of said output laser pulses to the predetermined energy for
industrial lithographic processing.
25. The method of claim 24, further comprising the step of reducing
an attenuation amount as optics of the laser resonator age to
maintain the energy of the output laser pulses at the predetermined
energy.
26. The method of claim 24, further comprising the step of reducing
an attenuation amount for producing output laser pulses having a
higher energy than the predetermined energy.
27. The method of claim 24, wherein the attenuating step is
performed outside of the laser resonator to outcoupled pulses from
the resonator.
28. The method of claim 27, wherein the attenuating step is
performed using an attenuator disposed within a housing, and the
method further comprises the step of flushing the housing with an
inert gas.
29. The method of claim 27, further comprising the step of driving
the attenuator with a motor for adjusting the attenuation to
maintain the energy of the output pulses at the predetermined
energy.
Description
PRIORITY
[0001] This application claims the benefit of priority to U.S.
provisional patent application Ser. No. 60/178,620, filed Jan. 27,
2000.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to lasers, and particularly high
repetition rate, line narrowed excimer and molecular fluorine
lasers for operation at specified pulse energies.
[0004] 2. Discussion of the Related Art
[0005] Semiconductor manufacturers are currently using deep
ultraviolet (DUV) lithography tools based on KrF-excimer laser
systems operating around 248 nm, as well as the following
generation of ArF-excimer laser systems operating around 193 nm.
Vacuum UV (VUV) lithography uses the molecular fluorine (F2) laser
operating around 157 nm.
[0006] Higher energy, higher efficiency excimer and molecular
fluorine lasers are being developed as lithographic exposure tools
for producing very small structures as chip manufacturing proceeds
deeper into the sub-0.18 micron regime. Specific characteristics of
laser systems sought to be improved upon particularly for the
lithography market include higher repetition rates, increased
energy stability and dose control, increased lifetimes of optical
components, increased percentage of laser system operation
"uptime", narrower output emission linewidths, improved wavelength
and bandwidth calibration and stability, and improved compatibility
with stepper/scanner imaging systems.
[0007] It is important for their respective applications to the
field of sub-quarter micron silicon processing that each of the
above laser systems become capable of emitting a narrow spectral
band of known bandwidth and around a very precisely determined and
finely adjustable absolute wavelength. Techniques for reducing
bandwidths by special resonator designs to less than 100 pm for use
with all-reflective optical imaging systems, and for catadioptric
imaging systems to less than 0.6 pm, are being continuously
improved upon.
[0008] A line-narrowed excimer or molecular fluorine laser used for
microlithography provides an output beam with specified narrow
spectral bandwidth. Narrowing of the bandwidth is generally
achieved through the use of a bandwidth narrowing and/or wavelength
selection and wavelength tuning module (hereinafter "line-narrowing
module") including most commonly prisms, diffraction gratings and,
in some cases, optical etalons. The line-narrowing module typically
functions to disperse incoming light angularly such that light rays
of the beam with different wavelengths are reflected at different
angles. Only those rays fitting into a certain "acceptance" angle
of the resonator undergo further amplification, and eventually
contribute to the output of the laser system.
[0009] Parameters of the line-narrowing module such as the
magnitude of angular dispersion, reflectivity for specific
wavelengths, linearity (i.e. absence of wavefront distortions),
scattering of the beam, etc., will thus affect the performance of
the laser. The optical components, i.e., prisms, gratings, etalons,
etc., of the line-narrowing module undergo changes that reduce
their performance with prolonged energy doses of laser power. This
reduced performance of the line-narrowing module is attributed to
"aging" of the optical components, wherein particularly the optical
surfaces take on defects resulting in reduced efficiency.
[0010] Line-narrowed excimer and molecular fluorine lasers are
particularly required to emit pulses of predetermined energy for
precise industrial processing. The energy of the output laser beam
depends on the composition and quality of the gas mixture, the
input energy to the discharge and the efficiency of the laser
resonator including the line-narrowing optics. The gas mixture is
typically replenished online using careful monitoring and gas
control procedures and the input energy to the discharge is
processor-controlled typically using an expert control system. The
initial efficiency of the resonator can be optimized by careful
selection and arrangement of line-narrowing components. However, as
the optical components and the laser tube itself age, the
efficiency of the laser decreases steadily from its initial optimum
state.
[0011] It is recognized herein that the pulse energy can be
maintained at the required level, when laser efficiency is reduced
by aging, by increasing the driving discharge voltage, as
illustrated at FIG. 1. The range .DELTA.E represents the pulse
energy range within which the operating laser is required to emit
laser pulses (the extent of .DELTA.E is exaggerated for
illustration in FIG. 1). The range .DELTA.HV represents the voltage
range within which the input discharge voltage may be adjusted in
order to produce the desired pulse energies. An absolute limitation
of .DELTA.HV is imposed by physical constraints on the components
of the gas discharge laser, while a practical limitation may be
imposed by laser performance considerations (see U.S. patent
application Ser. No. 60/171,717, which is assigned to the same
assignee and is hereby incorporated by reference).
[0012] Referring to FIG. 1, curve 1 represents the energy versus
driving voltage curve for a laser having a new line-narrowing
module and laser tube, or a line-narrowing module and laser tube
wherein the components, particularly the optics, have not yet aged.
Curves 2-4 are energy versus driving voltage curves for laser
systems having line-narrowing modules at various aging states from
less aging to more aging.
[0013] When components such as the optics and the laser tube are
new, and curve 1 is the relevant E-V curve, the driving voltage can
be adjusted to as low as HV.sub.min and the desired pulse energies
are achieved. The highest available pulse energy of the laser
system is E.sub.max, as shown, wherein the discharge voltage is set
at HV.sub.max and the laser is operating with new optics according
to curve 1, even though E.sub.max would lie outside the acceptable
range of pulse emission energies. When the optics are aged such the
laser is operating according to curve 2, application of only
HV.sub.min to the discharge will no longer achieve the desired
pulse energies. However, by increasing the input voltage, pulse
energies in the desired range .DELTA.E are easily achieved. At a
further stage of aging of the optics, the laser operates along
curve 3 when only input voltages at or near HV.sub.max will produce
the desired pulse energies. When the optics have aged further such
the laser is operating according to curve 4, the desired pulse
energies are no longer achievable.
[0014] Before the aging stage is reached wherein the laser operates
according to curve 4, the line-narrowing module is typically
replaced with a new one so that the laser can again operate
according to curve 1, and the process repeats itself. The duration
between when the laser operates along curve 1 with new optics and
when the laser can no longer produce pulse energies in the desired
range .DELTA.E is known as the "lifetime" of the line-narrowing
module, or the lifetime of the optics. At some point, the laser
tube itself ages such that even with a new line-narrowing module,
the laser cannot produce the desired pulse energies. At this point,
the laser tube has reached the end of its "lifetime".
[0015] Replacing the line-narrowing module implies servicing and
can result in undesirable service cost and downtime of the laser
system. It is desired to increase the lifetime of the
line-narrowing module and optics.
SUMMARY OF THE INVENTION
[0016] It is therefore an object of the invention to provide an
excimer or molecular fluorine laser system having a line-narrowing
module and/or other optical components with increased lifetimes
over conventional systems.
[0017] In accord with the above object, a laser system,
particularly an excimer or molecular fluorine laser system,
includes a laser tube filled with a gas mixture and having a
plurality of electrodes therein connected to a discharge circuit
for energizing the gas mixture, and a resonator including a
line-narrowing or line-selection module for generating a
line-narrowed laser beam. The laser system includes an attenuator
module for attenuating the pulse energy of the laser beam. The
preferred attenuator module is disposed outside of the resonator
for reducing the pulse energy of the output beam.
[0018] The laser system is preferably configured such that the
operating discharge voltage range .DELTA.HV is higher than for
conventional systems, such that the discharge circuit applies
higher energy electrical pulses to the discharge than conventional
laser systems, and particularly such that when the line-narrowing
module is new, the minimum achievable pulse energy is above a
desired pulse energy, i.e., when the discharge voltage is set to
HV.sub.min. The energy pulses output from the laser resonator are
attenuated by the attenuator module to the desired energy.
[0019] The attenuator module is preferably configured to exhibit
variable attenuation, and is particularly preferred to exhibit such
variable attenuation in steps. Thus, as the optics of the
line-narrowing module and/or the laser tube age, the attenuation is
reduced accordingly so that the laser system continues to achieve
the desired pulse energies.
[0020] The attenuator module may comprise a series of partially
reflective plates, preferably each being arranged at Brewster's
angle, each having a different reflectivity coefficient. The
attenuator module is adjustable so that one of the plates may be
selected that corresponds to the attenuation desired depending on
the aging stage of the line-narrowing module and/or the laser
tube.
[0021] The attenuator module may comprise one or more plates each
having a coating for reflecting a unique percentage of incident
light of a particular polarization, such as .pi.-polarized light.
The resonator of the laser is configured to output a substantially
polarized beam, such as exhibiting .pi.-polarization upwards of
95%-97%. The .pi.-polarization of the beam may be produced by using
Brewster surfaces such as the windows on the laser tube or a prism
surface or otherwise. The polarization of the beam may be produced
using a polarization plate in the resonator.
[0022] Advantageously, the effective lifetime of the line-narrowing
module and/or the laser tube is increased by using a high
attenuation when the line-narrowing module and/or laser tube is
new, and a decreasing attenuation percentage as the line-narrowing
optics and/or laser tube age. Further, operation at higher energies
than is typically desired is possible when the line-narrowing
optics are particularly new, such as by reducing the attenuation
percentage.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 shows various plots of pulse energy versus discharge
voltage corresponding to various stages of aging of the optics of
the line-narrowing module of a conventional laser system.
[0024] FIG. 2 is a flow chart showing the steps of preferred
methods for increasing the lifetime of a line-narrowing module of a
laser system and/or for producing higher output pulse energies each
in accord with preferred embodiments.
[0025] FIG. 3 shows various plots of pulse energy versus discharge
voltage corresponding to various stages of aging of the optics of
the line-narrowing module and/or laser tube of a laser system in
accord with a preferred embodiment.
[0026] FIG. 4 schematically shows a first setup for attenuating the
pulse energy of a laser beam in accord with a preferred
embodiment.
[0027] FIG. 5 schematically shows a second setup for adjustably
attenuating the pulse energy of a laser beam in accord with a
preferred embodiment.
[0028] FIG. 6 schematically shows a third setup for attenuating the
pulse energy of a laser beam and compensating a beam alignment in
accord with a preferred embodiment.
[0029] FIG. 7 schematically shows an excimer or molecular fluorine
laser system in accord with a preferred embodiment.
INCORPORATION BY REFERENCE
[0030] Each reference cited in the detailed description of the
preferred embodiment herein is, in addition to those references
cited above and below herein, including that which is described as
background, and the above invention summary, are hereby
incorporated by reference into the detailed description of the
preferred embodiment below, as disclosing alternative embodiments
of elements or features of the preferred embodiments not otherwise
set forth in detail below. A single one or a combination of two or
more of these references may be consulted to obtain a variation of
the preferred embodiments described in the detailed description
below. Further patent, patent application and non-patent references
are cited in the written description and are also incorporated by
reference into the detailed description of the preferred embodiment
with the same effect as just described.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] FIG. 2 is a flow chart showing steps of first and second
preferred methods, respectively, for increasing the lifetime of a
line-narrowing module and/or the laser tube of a laser system and
for producing higher output pulse energies. The first step S1 shown
in FIG. 2 is to configure a laser system for outputting pulse
energies in a range above a predetermined output pulse energy for
industrial processing.
[0032] High voltage power supplies connected to solid state pulser
modules are configured to apply a voltage to the electrodes within
a discharge chamber of a laser so that the laser produces output
pulses at a predetermined energy E.sub.0 (typically a few
millijoules for excimer and molecular fluorine lithography gas
discharge lasers). Preferred discharge circuits and electrodes are
described U.S. patent applications Ser. Nos. 60/128,227,
09/692,265, 60/149,392, 60/184,705, 60/204,095, 09/390,146, and
09/247,887 and U.S. Pat. Nos. 6,020,723 and 6,005,880, each of
which is assigned to the same assignee as the present application,
and at U.S. Pat. Nos. 5,729,565 and 4,860,300, all of the above
patents and patent application being hereby incorporated by
reference.
[0033] The predetermined energy E.sub.0 is specified by the
industrial processing that the laser is to be used for. The
discharge circuit is then configured to operate to apply a voltage
to the electrodes of the laser system within a range of voltages,
such as HV.sub.min<HV<HV.s- ub.max, where
.DELTA.HV=HV.sub.max-HV.sub.min. The range of applied voltages can
produce pulse energies in a range E.sub.min<E.sub.0<E.-
sub.max, where .DELTA.E=E.sub.max-E.sub.min, wherein the boundary
values E.sub.min and E.sub.max on the energy range .DELTA.E depend
on such factors as the age of the optics, the laser tube and the
gas mixture of the laser. A conventional laser system is configured
such that when the resonator optics, and particularly the
line-narrowing module and/or the laser tube, are new and have not
aged, the energy of output pulses is around E.sub.0 when the
discharge circuit operates around HV.sub.min, as discussed
above.
[0034] In contrast, the energy of output pulses is above E.sub.0
when the discharge circuit of the laser system in accord with the
preferred embodiment operates around HV.sub.min. The second step S2
shown in FIG. 2 is attenuating the output pulse energy to achieve
the predetermined pulse energy E.sub.0 specified for its intended
industrial processing use. Thus, the laser system is preferably
configured such that it is not capable of emitting output pulses of
energy E.sub.0 without attenuating the pulses in accord with step
S2.
[0035] Step S3a shown in FIG. 2 is to reduce the attenuation
percentage as the optics of the laser resonator, and particularly
of the line-narrowing module and/or the laser tube, age, to produce
the predetermined pulse energy E.sub.0. Advantageously, after a
certain amount of laser operation, the percentage of attenuation is
reduced as the optics and/or the laser tube age in accord with a
preferred method. After the same amount of laser operation, service
engineers are replacing line-narrowing modules of conventional
laser systems resulting in excessive laser system downtime and
cost.
[0036] With particular respect to lithography laser systems such as
excimer lasers, e.g., KrF and ArF lasers, or molecular fluorine
(F.sub.2) lasers, reducing laser downtime for servicing is
advantageous. Lithographic processing systems for producing
features on semiconductor wafers are typically desired to operate
continuously for having a largest throughput possible. Greater
throughput means greater commercial advantage to the chip makers
who use the laser system in their lithography systems together with
an imaging system and a stepper/scanner machine. Greater throughput
is advantageously realized according to the preferred embodiment
because optics of the laser system, and particularly the
line-narrowing module that typically includes refractive optics
that age due to exposure to a DUV or VUV laser beam, are replaced
less often, and thus the laser system is taken down less often to
perform the replacement.
[0037] Another advantage of the preferred embodiment is that the
driving voltage HV may be maintained around a particular value, and
only varied within a small range based on pulse-to-pulse or moving
average pulse energy stability or burst overshoot control
considerations, rather than making large HV range changes based on
aging of optics. The attenuation may be reduced rather than
increasing HV on average due to the aging. Advantageously, the
average value of HV can be relatively constant, and parameters that
may vary with HV will not be varied other than in a localized
pulse-to-pulse sense, or over a far smaller range than its
operating capable range, while the lifetime of the optics and/or
laser tube and/or gas mixture is/are increased according to
controlling the attenuation according to the preferred
embodiments.
[0038] Step S3b shown in FIG. 2 is reducing the attenuation
percentage when pulses having higher energy than the predetermined
energy E.sub.0 are desired for industrial or other use.
Advantageously, the maximum energy E'.sub.max achievable with the
laser system of the preferred embodiment is greater than the
maximum energy E.sub.max achievable with conventional laser
systems.
[0039] FIG. 3 illustrates the advantages of the laser system in
accord with a preferred embodiment, particularly when contrasted
with FIG. 1. FIG. 3 shows various plots of pulse energy versus
discharge voltage corresponding to various stages of aging of the
optics of the line-narrowing module and/or of the laser tube of a
laser system in accord with the preferred embodiment.
[0040] As shown in FIG. 3, curve 1 represents the energy versus
driving voltage of the laser system of the preferred embodiment
when the optics are new. Even when the driving voltage is set as
low as HV.sub.min, the predetermined pulse energy E.sub.0 (which
lies within the range .DELTA.E) cannot be produced without the
attenuation that is advantageously used to bring the energy within
the range .DELTA.E when desired.
[0041] The highest available pulse energy of the laser system is
E'.sub.max, as shown, wherein the discharge voltage is set at
HV.sub.max and the laser is operating with new optics according to
curve 1. The highest available energy E'.sub.max in accord with the
preferred embodiment, is shown to be above the highest available
energy E.sub.max of the conventional laser system by an amount
.DELTA.E.sub.max. Moreover, the attenuation may be set to reduce
the pulse energy to below the minimum energy achievable with the
conventional laser system, if desired. The laser system of the
preferred embodiment thus advantageously has a greater pulse energy
versatility than the conventional laser system.
[0042] When the optics and/or the laser tube are aged such that the
laser is operating according to curves 2-4 of FIG. 3, a lesser
amount of attenuation is used to produce the predetermined energy
E.sub.0. When the optics are aged such that the laser is operating
according to curves 5-6 of FIG. 3, then it is possible to achieve
the predetermined pulse energy E.sub.0 without using any
attenuation. Referring to FIG. 1, the conventional laser system
could not produce the predetermined energy E.sub.0 even when the
optics had aged only so that the laser system was operating
according to curve 4. Thus, the "lifetime" of the line-narrowing
module, the lifetime of the optics, or the lifetime of the laser
tube, or a combination thereof, of the laser system of the
preferred embodiment is advantageously increased.
[0043] FIG. 4 schematically shows a first preferred setup for
attenuating the pulse energy of a laser beam in accord with a
preferred embodiment. The following description of the preferred
setup is not intended to limit the scope of this application, and
is only illustrative, of arrangements that may be used for reducing
the energy of a laser beam outcoupled from a laser system, and
preferably an excimer or molecular fluorine laser system, to a
predetermined energy used preferably for an industrial application
and alternatively for a scientific, medical or other application
process. By attenuating the beam, the energy of the outcoupled beam
may be higher than is desired for the application process when the
laser optics, tube, etc. have not aged, such that when the laser
has aged even more than to an age when conventional laser system
would requirement servicing including replacement of one or more
optical components such as prisms, windows, etc., the
line-narrowing module or the laser tube, the laser of the preferred
embodiment may continue to be used for the application process.
[0044] Referring now to the illustrative example of FIG. 4, a laser
beam 2 is outcoupled from a laser resonator, such as that of an
excimer or molecular fluorine laser such as that which is preferred
and described with reference to FIG. 7, below. The beam 2 impinges
upon a reflective attenuator 4. Although not preferred, the
attenuator 4 may alternatively be an absorption attenuator, such as
to absorb the same percentage of incident light as the preferred
reflective attenuator 4 would reflect, or a combination of
reflection and absorption may be used. A desired percentage of the
intensity of the beam 2 is preferably reflected to the module 6,
and an attenuated beam 8 transmits through the attenuator 4 and
continues on toward an industrial processing application.
[0045] The attenuator plate 4 can be arranged at Brewster's angle
to have only a coating on one side of the plate, while the other
side is uncoated and doesn't influence a .pi.-polarized laser. In
this case, the degree of .pi.-polarization of the laser beam 8 can
be increased over that of the beam 2.
[0046] The attenuator 4 has a certain reflectivity. The attenuator
4 may be replaced with another one having a different reflectivity
depending on the amount of attenuation that is desired. The laser
beam may be polarized and the attenuator 4 may in that case be
configured to reflect a certain percentage of the polarization
component of the laser. The laser may be polarized using Brewster
surfaces such as the laser tube windows, prism surfaces or
otherwise. A polarization plate may be inserted into the resonator,
as well. The outcoupling of the beam 2 may be polarization coupled
at an angled optical surface within the resonator or through a
partially reflective resonator reflector. The attenuator 4 may have
a coating for reflecting a certain percentage of the laser beam 2,
such as a polarization sensitive reflection coating.
[0047] Several attenuators may be selectively interchanged
depending on the amount of attenuation desired. For example,
attenuators having reflection coefficients of 40%, 30%, 20% and 10%
may be readily available to the operator of the laser system, and
may be interchanged after predetermined amounts of laser operation
and corresponding aging, and a processor may control the laser
output energy according to the attenuator being used.
[0048] The module 6 may be a beam dump, or it may be a diagnostic
module such as a pulse energy, power or array detector to monitor
the pulse energy, moving average energy or power, temporal pulse
shape or spatial profile, beam alignment, divergence or spectral
distribution, wavelength, or bandwidth of the beam or other beam
parameter, and the module 6 may include an inspection window. Thus,
the attenuator 4 may serve the dual role of attenuating the beam 2
and reflecting part of the beam 2 to a diagnostic module of the
module 6, shown in FIG. 4. Uses of such a diagnostic module may be
as described at U.S. patent applications Ser. Nos. 09/447,882,
09/416,344, 09/379,034, 60/166,854, 09/271,020, 09/162,424,
60/166,952 and/or 09/532,276, each of which is assigned to the same
assignee as the present application, and is hereby incorporated by
reference.
[0049] The attenuation may alternatively be controlled by adjusting
a trace amount of a gas additive to the gas mixture such as
preferably xenon (see U.S. patent application Ser. No. 09/513,025,
assigned to the same assignee as the present application and hereby
incorporated by reference), or argon, krypton, oxygen, HCl,
CO.sub.2, CF.sub.4, or N.sub.2. A gas control unit would adjust the
amount of the gas additive according to the amount of attenuation
desired, in accord with the aging stage of the optics.
[0050] FIG. 5 schematically shows a second preferred setup for
adjustably attenuating the pulse energy of a laser beam in accord
with a preferred embodiment. Again, the beam 2 is outcoupled from a
laser resonator. The beam impinges upon an attenuation unit 10
including multiple attenuation plates 12. Each attenuation plate 12
corresponds to a different degree of attenuation, such as 10%-40%
as described above. One skilled in the art would realize that
attenuation plates above 40% and at lower intervals than 10% may be
used, and that more or less than four may be provided.
[0051] The attenuation unit 10 has a face 14 that includes the
plates 12. The face 14 is rotatable so that the plate 12 having the
attenuation characteristics desired may be "clicked" into place in
the path of the beam 2. The plates 12 may also be set up in a
modified attenuation unit to slide into place individually, or to
rotate like a revolving door selectively into place.
[0052] FIG. 6 schematically shows an arrangement according to a
preferred embodiment including the attenuator plate 4 and module 6
including a diagnostic module and/or beam dump similar to those
shown and described with respect to FIG. 4, above. As can be seen
in FIG. 6, an alignment of the beam 2 is moved a distance .DELTA.s
from its original path as a consequence of traversing the
attenuator 4. In order to move the beam back to its original path,
a compensator plate 16 according to a preferred embodiment is
inserted after the attenuator 4, and may alternatively be disposed
before the attenuator 4. A same or a different compensator may be
configured to be used together with each attenuator 12 of the
variable attenuator design of FIG. 5.
[0053] Referring now to FIG. 7, the preferred excimer or molecular
fluorine laser system will now be described. A gas discharge laser
system, preferably a DUV or VUV laser system, such as an excimer
laser, e.g., ArF or KrF, or a molecular fluorine (F2) laser system
for deep ultraviolet (DUV) or vacuum ultraviolet (VUV) lithography,
is schematically shown at FIG. 7. Alternative configurations for
laser systems for use in such other industrial applications as TFT
annealing and/or micromachining, e.g., are understood by those
skilled in the art as being similar to and/or modified from the
system shown in FIG. 7 to meet the requirements of that
application. For this purpose, alternative DUV or VUV laser system
and component configurations are described at U.S. patent
applications Ser. Nos. 09/317,695, 09/130,277, 09/244,554,
09/452,353, 09/317,527, 09/343,333, 60/122,145, 60/140,531,
60/162,735, 60/166,952, 60/171,172, 60/141,678, 60/173,993,
60/166,967, 60/147,219, 60/170,342, 60/162,735, 60/178,445,
60/166,277, 60/167,835, 60/171,919, 09/640,595, 60/202,564,
60/204,095, 60/172,674, 09/574,921 and 60/181,156, and U.S. Pat.
Nos. 6,005,880, 6,061,382, 6,020,723, 5,946,337, 6,160,832,
6,160,831, 6,157,662, 6,154,470, 6,014,206, 5,559,816, 4,611,270,
5,761,236, 4,393,505, 4,905,243, 6,081,542, 6,061,382, 4,916,707,
and 4,977,573 each of which is assigned to the same assignee as the
present application, and those references set forth above, are
hereby incorporated by reference.
[0054] The system shown in FIG. 7 generally includes a laser
chamber 202 having a pair of main discharge electrodes 46, 48,
e.g., as referred briefly to above, connected with a solid-state
pulser module 204, and a gas handling module 206. The solid-state
pulser module 204 is powered by a high voltage power supply 208.
The laser chamber 202 is surrounded by optics module 210 and optics
module 212, forming a resonator. The optics module 210 is
preferably controlled by an optics control module 214, or may be
alternatively directly controlled by a computer 216, and the front
optics module 212 is preferably controlled by the control unit 70
described above, which may be a part of or separate from the module
214.
[0055] The computer 216 for laser control receives various inputs
and controls various operating parameters of the system. A
diagnostic module 218 receives and measures one or more parameters
of a split off portion of the main beam 220 via optics for
deflecting a small portion of the beam toward the module 218, such
as preferably a beam splitter module 222, as shown. The beam 220 is
preferably the laser output to an imaging system (not shown) and
ultimately to a workpiece (also not shown), and may be output
directly to an application process. In accord with the preferred
embodiments described above, the beam 220 is the same as the beam 2
of FIGS. 4-6 and is attenuated by the attenuator 4 or 10 before
proceeding to the imaging system or application process. The laser
control computer 216 communicates through an interface 224 with a
stepper/scanner computer 226 and other control units 228.
Laser Chamber
[0056] The laser chamber 202 contains a laser gas mixture and
includes one or more preionization electrodes (not shown) in
addition to the pair of main discharge electrodes 46, 48. Preferred
main electrodes 46 and 48 are described at U.S. patent applications
Ser. Nos. 09/453,670 and 60/184,705, each of which is assigned to
the same assignee as the present application and is hereby
incorporated by reference. Other electrode configurations are set
forth at U.S. Pat. Nos. 5,729,565 and 4,860,300, each of which is
assigned to the same assignee, and alternative embodiments are set
forth at U.S. Pat. Nos. 4,691,322, 5,535,233 and 5,557,629, all of
which are hereby incorporated by reference. Preferred preionization
units are described at U.S. patent application Ser. No. 09/692,265
and details and alternative configurations are additionally set
forth at U.S. patent applications Ser. Nos. 60/127,237, 09/532,276
and 09/247,887, each of which is assigned to the same assignee as
the present application, and alternative embodiments are set forth
at U.S. Pat. Nos. 5,337,330, 5,818,865 and 5,991,324, all of the
above patents and patent applications being hereby incorporated by
reference.
Power Supply Circuit and Pulser Module
[0057] The solid-state pulser module 204 and high voltage power
supply 208 supply electrical energy in compressed electrical pulses
to the preionization and main electrodes 46, 48 within the laser
chamber 202 to energize the gas mixture. Components of the
preferred pulser module and high voltage power supply may be
described at U.S. patent applications Ser. Nos. 60/149,392,
60/198,058, 60/204,095, 09/432,348 and 09/390,146, and 60/204,095,
and U.S. Pat. Nos. 6,005,880 and 6,020,723, each of which is
assigned to the same assignee as the present application and which
is hereby incorporated by reference into the present application.
Other alternative pulser modules are described at U.S. Pat. Nos.
5,982,800, 5,982,795, 5,940,421, 5,914,974, 5,949,806, 5,936,988,
6,028,872 and 5,729,562, each of which is hereby incorporated by
reference. A conventional pulser module may generate electrical
pulses in excess of one Joule of electrical power (see the '988
patent, mentioned above).
Laser Resonator
[0058] The laser resonator which surrounds the laser chamber 202
containing the laser gas mixture includes optics module 210
including line-narrowing optics for a line narrowed excimer or
molecular fluorine laser, which may be replaced by a high
reflectivity mirror or the like in a laser system wherein either
line-narrowing is not desired, or if line narrowing is performed at
the front optics module 212, or a spectral filter external to the
resonator is used, or if the line-narrowing optics are disposed in
front of the HR mirror, for narrowing the bandwidth of the output
beam.
[0059] The laser chamber 202 is sealed by windows transparent to
the wavelengths of the emitted laser radiation 220. The windows may
be Brewster windows or may be aligned at another angle, e.g.,
5.degree., to the optical path of the resonating beam. One of the
windows may include the interferometer 60 described above which
also serves to output couple the beam.
Extra-Resonator Features
[0060] After a portion of the output beam 220 passes the outcoupler
of the optics module 212, that output portion impinges upon beam
splitter module 222 which includes optics for deflecting a portion
of the beam to the diagnostic module 218, or otherwise allowing a
small portion of the outcoupled beam to reach the diagnostic module
218, while a main beam portion 220 is allowed to continue as the
output beam 220 of the laser system. The beam splitter 222 may be
the same or a different component from the attenuator 4 or 10 of
the preferred embodiments described above. The diagnostic module
218 may include a photodiode 68 and/or a monitor etalon 82, and may
be the same or a different component from the module 6, described
above.
[0061] Preferred optics include a beamsplitter or otherwise
partially reflecting surface optic. The optics may also include a
mirror or beam splifter as a second reflecting optic. More than one
beam splitter and/or HR mirror(s), and/or dichroic mirror(s) may be
used to direct portions of the beam to components of the diagnostic
module 218. A holographic beam sampler, transmission grating,
partially transmissive reflection diffraction grating, grism, prism
or other refractive, dispersive and/or transmissive optic or optics
may also be used to separate a small beam portion from the main
beam 220 for detection at the diagnostic module 218, while allowing
most of the main beam 220 to reach an application process directly
or via an imaging system or otherwise.
[0062] The output beam 220 may be transmitted at the beam splitter
module 222 while a reflected beam portion is directed at the
diagnostic module 218, or the main beam 220 may be reflected, while
a small portion is transmitted to the diagnostic module 218. The
portion of the outcoupled beam which continues past the beam
splitter module 222 is the output beam 220 of the laser, which
propagates toward an industrial or experimental application such as
an imaging system and workpiece for photolithographic applications.
Variations of beam splitter modules 222 particularly for a
molecular fluorine laser system are set forth at U.S. patent
applications Nos. 09/598,552, 60/177,809 and 60/140,530, which are
each assigned to the same assignee as the present application and
are hereby incorporated by reference.
Beam Path Enclosure
[0063] Also particularly for the molecular fluorine laser system,
and for the ArF laser system, an enclosure (not shown) may seal the
beam path of the beam 220 such as to keep the beam path free of
photoabsorbing species. Smaller enclosures may seal the beam path
between the chamber 202 and the optics modules 210 and 212 and
between the beam splitter module 222, which itself may be within
the same or a separate enclosure, and the diagnostic module 218.
The preferred enclosure is described in detail in U.S. patent
applications Ser. Nos. 09/343,333, 09/598,552, 09/594,892,
09/131,580 and 60/140,530, each of which is assigned to the same
assignee and is hereby incorporated by reference, and U.S. Pat.
Nos. 5,559,584, 5,221,823, 5,763,855, 5,811,753 and 4,616,908, all
of which are hereby incorporated by reference.
Diagnostic Module
[0064] The diagnostic module 218 preferably includes at least one
energy detector. This detector measures the total energy of the
beam portion that corresponds directly to the energy of the output
beam 220 (see U.S. Pat. No. 4,611,270 and U.S. patent application
Ser. No. 09/379,034, each of which is assigned to the same assignee
and is hereby incorporated by reference). An optical configuration
such as an optical attenuator, e.g., a plate or a coating, or other
optics may be formed on or near the detector or beam splitter
module 222 to control the intensity, spectral distribution and/or
other parameters of the radiation impinging upon the detector (see
U.S. patent applications Ser. Nos. 09/172,805, 60/172,749,
60/166,952 and 60/178,620, each of which is assigned to the same
assignee as the present application and is hereby incorporated by
reference).
[0065] One other component of the diagnostic module 218 is
preferably a wavelength and/or bandwidth detection component such
as a monitor etalon or grating spectrometer (see U.S. patent
applications Ser. Nos. 09/416,344, 60/186,003, 60/158,808,
60/186,096, 60/186,096 and 60/186,096 and 60/202,564, each of which
is assigned to the same assignee as the present application, and
U.S. Pat. Nos. 4,905,243, 5,978,391, 5,450,207, 4,926,428,
5,748,346, 5,025,445, and 5,978,394, all of the above wavelength
and/or bandwidth detection and monitoring components being hereby
incorporated by reference. This monitor etalon can be the same one
described above with respect to FIGS. 9a and 9b, or a second
monitor etalon. The spectrometer may be within a temperature and
pressure controlled housing such as is described in the U.S. Pat.
No. 60/158,808 application.
[0066] Other components of the diagnostic module may include a
pulse shape detector or ASE detector, such as are described at U.S.
patent applications Ser. Nos. 09/484,818 and 09/418,052,
respectively, each of which is assigned to the same assignee as the
present application and is hereby incorporated by reference, such
as for gas control and/or output beam energy stabilization, or to
monitor the amount of amplified spontaneous emission (ASE) within
the beam to ensure that the ASE remains below a predetermined
level, as set forth in more detail below. There may be a beam
alignment monitor, e.g., such as is described at U.S. Pat. No.
6,014,206 which is assigned to the same assignee and is hereby
incorporated by reference.
Control Processor
[0067] The processor or control computer 216 receives and processes
values of some of the pulse shape, energy, ASE, energy stability,
energy overshoot for burst mode operation, wavelength, spectral
purity and/or bandwidth, among other input or output parameters of
the laser system and output beam. The processor 216 also controls
the line narrowing module to tune the wavelength and/or bandwidth
or spectral purity, and controls the power supply and pulser module
204 and 208 to control preferably the moving average pulse power or
energy, such that the energy dose at points on the workpiece is
stabilized around a desired value. In addition, the computer 216
controls the gas handling module 206 which includes gas supply
valves connected to various gas sources.
[0068] Further details of the control processor 216 such as for
performing burst overshoot control and controlling the gas supply
unit by monitoring total input energy to the discharge, among other
parameters, for determining the timing and amounts of gas
replenishment actions, are described at U.S. patent application
Ser. No. 09/588,561, which is assigned to the same assignee as the
present application and is hereby incorporated by reference.
Gas Mixture
[0069] The laser gas mixture is initially filled into the laser
chamber 202 during new fills. The gas composition for a very stable
excimer or molecular fluorine laser in accord with the preferred
embodiment uses helium or neon or a mixture of helium and neon as
buffer gas(es), depending on the particular laser being used.
Preferred gas compositions are described at U.S. Pat. Nos.
4,393,405 and 4,977,573 and U.S. patent applications Ser. Nos.
09/317,526, 09/513,025, 60/124,785, 09/418,052, 60/159,525 and
60/160,126, each of which is assigned to the same assignee and is
hereby incorporated by reference into the present application. The
concentration of the fluorine in the gas mixture may range from
0.003% to 1.00%, and is preferably around 0.1%. An additional gas
additive, such as a rare gas, such as xenon, may be added for
increased energy stability and/or as an attenuator as described in
the U.S. Pat. No. 09/513,025 application incorporated by reference
above. Specifically, for the F2-laser, an addition of xenon and/or
argon may be used. The concentration of xenon or argon in the
mixture may range from 0.0001% to 0.1%. For an ArF-laser, an
addition of xenon or krypton may be used also having a
concentration between 0.0001% to 0.1%. For the KrF laser, an
addition of xenon or argon may be used also having a concentration
between 0.0001% to 0.1%.
Gas Replenishment, General
[0070] Halogen and rare gas injections, total pressure adjustments
and gas replacement procedures are performed using the gas handling
module 206 preferably including a vacuum pump, a valve network and
one or more gas compartments. The gas handling module 206 receives
gas via gas lines connected to gas containers, tanks, canisters
and/or bottles. Some prefered and alternative gas handling and/or
replenishment procedures are described at U.S. Pat. Nos. 4,977,573
and 5,396,514 and U.S. patent applications Nos. 60/124,785,
09/418,052, 09/379,034, 60/159,525, 60/171,717, and 60/159,525,
each of which is assigned to the same assignee as the present
application, and U.S. Pat. Nos. 5,978,406, 6,014,398 and 6,028,880,
all of which are hereby incorporated by reference. A xenon gas
supply may be included either internal or external to the laser
system according to the '025 application, mentioned above.
Line-Narrowing
[0071] A general description of the line-narrowing features of the
preferred embodiment is provided here, followed by a listing of
patent and patent applications being incorporated by reference as
describing variations and features that may used with the preferred
embodiments described above for providing an output beam with a
high spectral purity or bandwidth (e.g., below 0.6 pm). Exemplary
line-narrowing optics contained in the optics module 210 include a
beam expander, an optional etalon and a diffraction grating, which
produces a relatively high degree of dispersion, for a narrow band
laser such as is used with a refractive or catadioptric optical
lithography imaging system. As referred to above, the front optics
module 212 may include line-narrowing optics (e.g., outcoupling
interferometer, birefringent plate, grating, grism) as well (see
the U.S. Pat. Nos. 60/166,277, 60/173,993 and 60/166,967
applications, each being assigned to the same assignee and hereby
incorporated by reference).
[0072] The beam expander of the above exemplary line-narrowing
optics of the optics module 210, and that of the embodiment
described above in front of the output coupling interferometer 60,
preferably includes one or more prisms. The beam expander may
include other beam expanding optics such as a lens assembly or a
converging/diverging lens pair. The grating or a highly reflective
mirror is preferably rotatable and in Littrow configuration so that
the wavelengths reflected into the acceptance angle of the
resonator can be selected or tuned. Alternatively, the grating, or
other optic or optics, or the entire line-narrowing module may be
pressure tuned, such as is set forth in the U.S. Pat. Nos.
60/178,445 and 09/317,527 applications, each of which is assigned
to the same assignee and is hereby incorporated by reference. The
grating may be used both for dispersing the beam for achieving
narrow bandwidths and also preferably for retroreflecting the beam
back toward the laser tube 202. Alternatively, a highly reflective
mirror may be positioned before or after the grating which receives
a reflection from the grating and reflects the beam back toward the
grating, such as in a Littman configuration, or the grating may be
a transmission grating. One or more dispersive prisms may also be
used, and more than one etalon may be used.
[0073] Depending on the type and extent of line-narrowing and/or
selection and tuning that is desired, and the particular laser that
the line-narrowing optics are to be installed into, there are many
alternative optical configurations that may be used. For this
purpose, those shown in U.S. Pat. Nos. 4,399,540, 4,905,243,
5,226,050, 5,559,816, 5,659,419, 5,663,973, 5,761,236, 6,081,542,
6,061,382 and 5,946,337, and U.S. patent applications Nos.
09/317,695, 09/130,277, 091715,803, 60/181,158, 60/218,417,
09/584,420, 60/212,257, 60/212,301, 60/215,933, 09/244,554,
09/317,527, 09/073,070, 60/124,241, 60/140,532, 60/147,219 and
60/140,531, 60/147,219, 60/170,342, 60/172,749, 60/178,620,
60/173,993, 60/166,277, 60/166,967, 60/167,835, 60/170,919,
60/186,096, each of which is assigned to the same assignee as the
present application, and U.S. Pat. Nos. 5,095,492, 5,684,822,
5,835,520, 5,852,627, 5,856,991, 5,898,725, 5,901,163, 5,917,849,
5,970,082, 5,404,366, 4,975,919, 5,142,543, 5,596,596, 5,802,094,
4,856,018, 5,970,082, 5,978,409, 5,999,318, 5,150,370 and
4,829,536, and German patent DE 298 22 090.3, are each hereby
incorporated by reference into the present application.
[0074] Optics module 212 preferably includes means for outcoupling
the beam 220, such as a partially reflective resonator reflector.
The beam 220 may be otherwise outcoupled such as by an
intra-resonator beam splitter or partially reflecting surface of
another optical element, and the optics module 212 would in this
case include a highly reflective mirror. The optics control module
214 preferably controls the optics modules 210 and 212 such as by
receiving and interpreting signals from the processor 216, and
initiating realignment or reconfiguration procedures (see the '241,
'695, '277, '554, and '527 applications mentioned above).
[0075] While exemplary drawings and specific embodiments of the
present invention have been described and illustrated, it is to be
understood that that the scope of the present invention is not to
be limited to the particular embodiments discussed. Thus, the
embodiments shall be regarded as illustrative rather than
restrictive, and it should be understood that variations may be
made in those embodiments by workers skilled in the arts without
departing from the scope of the present invention as set forth in
the claims that follow, and equivalents thereof.
[0076] In addition, in the method claims that follow, the steps
have been ordered in selected typographical sequences. However, the
sequences have been selected and so ordered for typographical
convenience and are not intended to imply any particular order for
performing the steps, except for those claims wherein a particular
ordering of steps is expressly set forth or understood by one of
ordinary skill in the art as being necessary.
* * * * *